1. Field of the Invention
The present invention relates to a vehicular steering system having a steering mechanism for steering the steerable road wheels of a vehicle in accordance with a value that is obtained by mechanically adding an auxiliary steering angle, which can be electrically controlled by an auxiliary steering angle superposition mechanism, to the steering wheel angle of a steering wheel steered by a driver. In particular, the invention relates to novel technical improvements for performing steering (intervention steering) to correct the steering operation of the steering wheel by the driver, and at the same time changing a transmission characteristic between the steering angle of the steering wheel operated by the driver and the steered angle of the steerable road wheels.
2. Description of the Related Art
Conventionally, there has been well known a vehicular steering system in which an auxiliary steering angle superposition mechanism and a steering mechanism are interposed between a steering wheel and steerable road wheels of a vehicle, so that the steerable road wheels are steered by mechanically superposing an amount of auxiliary steering by an electric motor in the auxiliary steering angle superposition mechanism on an amount of steering of the steering wheel operated by a driver. A planetary gear mechanism, a differential gear mechanism, a harmonic drive or the like is used as the auxiliary steering angle superposition mechanism.
In such a kind of vehicular steering system, there has also been proposed a technique that changes a transmission characteristic of the steered angle of the steerable road wheels with respect to the steering angle of the steering wheel (steering wheel angle) by the driver in accordance with the traveling condition of the vehicle (see, for example, a first patent document: Japanese patent No. 3518590).
In the conventional apparatus of the above-mentioned first patent document, the transmission characteristic between a steering wheel angle θh (steering angle of the steering wheel operated by the driver) and the steered angle of the steerable road wheels is determined based on the traveling condition of the vehicle such as the vehicle speed, the steering speed of the steering wheel, etc., and a target steered angle θpref is also determined based on the steering wheel angle θh and the transmission characteristic.
In addition, a target auxiliary steering angle θsref is determined based on a characteristic that is decided from the target steered angle θpref and the mechanical construction of the auxiliary steering angle superposition mechanism.
For example, in case where the auxiliary steering angle superposition mechanism is controlled to be driven based on the target steered angle θpref, a sensor for detecting the steered angle θp of the vehicle is used so that the auxiliary steering angle θs of the auxiliary steering angle superposition mechanism is controlled to be driven so as to satisfy the following expression (1).θpref−θp=0  (1)
Further, in case where the auxiliary steering angle superposition mechanism is controlled to be driven based on the target auxiliary steering angle θsref, a sensor for detecting the auxiliary steering angle θs is used so that the auxiliary steering angle θs of the auxiliary steering angle superposition mechanism is controlled to be driven so as to satisfy the following expression (2).θsref−θs=0  (2)
For example, a rotary encoder or the like is used as a sensor for detecting the steered angle θp or the auxiliary steering angle θs of the vehicle, as shown in the above-mentioned first patent document.
The rotary encoder outputs two-phase pulse signals comprising a combination of “0” and “1”, so the individual steered angle and auxiliary steering angle can be obtained by counting these pulse signals.
However, when the pulse signals become unable to be obtained due to a break or disconnection of either of signal lines for the two-phase pulses, failure of the rotary encoder, etc., normal counting of the pulse signals becomes impossible in spite of an actual change in the steered angle θp, so the detected value of the steered angle θp or the auxiliary steering angle θs does not change.
Thus, in case of using the steered angle θp or the auxiliary steering angle θs that does not change due to the failure, it will become impossible to make the expression (1) or expression (2) hold if the driving control of the auxiliary steering angle superposition mechanism is performed based on the expression (1) or expression (2).
Accordingly, the control of the auxiliary steering angle θs to be superposed by the auxiliary steering angle superposition mechanism becomes abnormal, and as a result, there is a possibility that the steerable road wheels might be steered in a direction not intended by the driver.
Thus, in the above-mentioned first patent document, in order to detect the break or disconnection of the signal lines and the failure of the rotary encoder, the steered angle θp is calculated from the steering wheel angle θh and the auxiliary steering angle θs, as shown by the following expression (3).θp=θh+θs  (3)
Whether the angle detection section is in failure is determined by comparing the steered angle θp obtained from the expression (3) with a steered angle of the steerable road wheels that is estimated based on a difference between the speeds of right and left road wheels.
However, according to such a determination method, failure can not be detected until when the steering in the direction not intended by the driver proceeds.
It is necessary to separately or independently detect the failure of the rotary encoder at an early time in order to solve the above-mentioned problem, but the rotary encoder has all the combinations of two phase signals of “0” and “1”, as stated above, so it is impossible to detect the failure of the rotary encoder from the correlation of the two-phase signals.
As described above, in the conventional vehicular steering system, particularly in the first patent document, in order to detect the break or disconnection of the signal lines or the failure of the rotary encoder, the steered angle θp is calculated from the steering wheel angle θh and the auxiliary steering angle θs, as shown by the expression (3), and compared with the steered angle estimated based on the difference between the right and left road wheel speeds, so there is a problem that failure can not be detected until when the steering in the direction not intended by the driver proceeds.
In addition, there is also another problem that even if the failure of the rotary encoder is intended to be separately or independently detected at an early time, detection signals of the rotary encoder include all the combinations of two-phase “0” and “1” signals and hence it is impossible to detect the failure of the rotary encoder from the correlation of the two-phase signals.